This invention relates to digital automatic gain control and especially to achieving speed sufficient for real-time signal processing.
Automatic gain control of a process variable is the adjustment of the system gain, a multiplier, so as to keep constant a long-term average of the value of a process variable. It has long been used in radio communication receivers to control the loudness of the output sound. Digital automatic gain control has advantages over analog methods but has been limited in use because the calculations involved have required more time than can be afforded in fast, real-time process control. In particular, the calculations have been thought to require multiplication and division steps, which are much slower than addition and subtraction. A method of calculation that avoids multiplication and division would make digital AGC feasible in areas of process control in which it is not feasible otherwise.
Well known analog methods of automatic gain control involve the averaging of the signal or process variable to be controlled by means of a low-pass filter and utilization of this average to adjust the system gain through some kind of level-adjusting component. With the introduction of the multiplying digital-to-analog converter (MDAC), it has become possible to achieve automatic gain control digitally. The digital form of low-pass filtering is known to be exponential averaging. The exponential average of a signal sampled repeatedly can be tracked over time by accumulating the difference between the current signal and the previous value of the average and dividing the result by a chosen time constant. The result is the updated value of the exponential average. To regulate that average to a desired level, the gain is adjusted proportionally as follows: new gain is to old gain as the desired level is to the current average. After any possible combination of steps, calculation of new gain by this method can be shown to require at least one division and one multiplication per sampling cycle.
One application of a fast digital AGC is in a Machine Tool Monitor to adjust a vibration signal to a chosen average level; the signal is then analyzed to detect acoustic signatures characteristic of tool breakage and initial touch of the cutting tool to a workpiece. A number of commonly assigned patents and copending applications cover various aspects of the MTM. Among the latter that are relevant to this invention and which disclose a digitally programmable multiplying digital-to-analog converter in the analog signal channel to preprocess the vibration signal prior to digitization are: Ser. No. 744,083, filed June 12, 1985 now U.S. Pat. No. 4,724,524, C. E. Thomas et al, "Vibration-Sensing Tool Break and Touch Detector Optimized for Machining Conditions" and Ser. No. 943,397, filed Dec. 19, 1986, now U.S. Pat. No. 4,764,760, J. F. Bedard et al, "Automatic Gain Control for Machine Tool Monitor". The latter has a hardware AGC. A software controlled AGC for a worn tool detector is in U.S. Pat. No. 4,514,797.